Knowledge Bank

The photoionization potential of adsorbed monolayers was measured on cadmium sulfide and pyrex glass. Using the dielectric constants of these substances, the vacuum ionization potential could be estimated within $\pm 0.1$ e.v. (ref. 1). The materials were evaporated on the substrates under nitrogen. A concentration of less than 50 mg liter$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ was used to insure monolayer coverage. The calculation was made with the Hartree-Fock selfconsistent field equations in the LCAO and $\pi$-electron approximations. Using Koopman's Theorem and a single determinant wave function, the ionization potentials could be estimated from the ground state problem. Adjusting the calculation by considering the approximations in using Koopman's Theorem improved the results. This was done in two ways. A `variable configuration method', similar to limited configuration interaction was developed. Then the interaction of the $\sigma$- and $\alpha$-electrons, during the ionization process, was considered. As a check on this method, open shell calculations were made and the ionization potential was determined from the difference between the total $\pi$-electron energies of the ground and singly ionized states. With a reasonable choie of the semi-empirical parameters, the calculated values come within 0.5 e.v. of the measured values.